Fans are used on a variety of equipment for cooling purposes and are frequently used to provide air flow through the cooling system of an internal combustion engine during its operation. Such fans can be quite large with diameters most frequently between 12 to 48 inches in diameter. Such fans are frequently formed from assemblies including a central metal hub adapted to be driven by the internal combustion engine or other mechanical or hydraulic means with a plurality of air-moving blades formed from an outer plastic body molded into engagement with the central metal hub, and such composite fans are frequently preferred because they are lighter, less expensive to manufacture and consume less power from the driving source.
Substantial forces are imposed on fans during their operation. The forces imposed on the fan blade surfaces as they are moved against and in the surrounding air generate substantial forces on and within the individual blades themselves and at their connection with the central driving hub. In addition, since such fans are rotated at speeds from several thousand revolutions per minute, substantial centrifugal forces are developed. In addition to the forces of air resistance and the centrifugal forces of rotation, forces are sporadically imposed on the fan by misfiring of the internal combustion engine's cylinders, drive train vibration, gusting winds and other unpredictable sources of force.
The resulting effect of such forces is particularly troublesome where it is desirable to form the blades themselves with molded plastic material. First, plastic materials, such as nylon, polypropylene and the like, are not as strong as metals such as steel and aluminum. Therefore, it is desirable to utilize design methods to improve the strength of the plastic body.
To improve the strength of bodies formed from plastic materials, it is common to incorporate into the body of the plastic materials fiber-reinforcement, such as fiber glass having fiber lengths as small as 20 thousandths of an inch and as long as manufacturing processes allow. Such fiber reinforced plastic materials are commonly available from suppliers such as DuPont. The inclusion of reinforcing fibers in parts formed from plastic materials improves their sensitivity to stress concentrations and increases the modulus of elasticity, elastic limit, and ultimate strength of the plastic materials; however, such materials still fail to provide the strength of metals and it remains more difficult to obtain a durable and reliable composite fan assembly with a plastic blade-forming element.
The problems of providing durable and reliable composite blade assemblies is compounded, not only by the internal stresses generated within the plastic blade-forming portion, but at the interface between the plastic blade-forming portion and the central driven hub. There is little or no adhesion between the peripheral plastic blade-forming portion and the central driven hub, and the interface between the peripheral plastic portion and the driven central hub must bear and survive the forces imposed on the peripheral blade-forming portion by its rotation, including the driving forces imposed at the interface by the driving mechanism and the air resistance on the blades and any shock loads that may be imposed during operation of the fan assembly.
U.S. Pat. No. 4,957,414 discloses a composite fan assembly including a central metal hub with a plurality of arms and a plastic fan structure molded to the periphery of the central hub. In the fan and hub assembly of U.S. Pat. No. 4,957,414, an attempt was made to increase the durability and reliability of the fan assembly by forming openings in the central metal hub and arms of predetermined sizes and positions to, not only, provide a driving interface between the central metal hub and the peripheral plastic portion, but to achieve a predetermined resonant frequency of the fan blades when molded to the hub. In addition, the arms of the central hub are formed with a smoothly curving profile, devoid of corners, to avoid the formation of stress points in the fan blades that are molded onto the central metal hub.